1. Field of the Invention
This invention relates to an inspection method and an inspection device, or apparatus. For example, the invention is suitable for an inspection method and an inspection method for inspecting defects on surfaces and inside of inspection objects such as semiconductor wafers, magnetic disks, liquid crystal substrates, mask substrates, and so forth.
2. Description of the Related Art
To monitor the dust occurring condition of production apparatuses in a production line of a semiconductor substrate (semiconductor wafer), the inspection of defects such as foreign matters adhering to the surface of the substrate, scratches generated during a processing, etc has so far been carried out. In the case of the semiconductor substrate before the formation of a circuit pattern, for example, fine foreign matters and defects having sizes of not greater than dozens of nm on their surface must be carried out. Besides the foreign matters and defects described above, crystal defects existing in a shallow region in the proximity of the substrate surface, surface roughness of the substrate and furthermore, materials that constitute the defect must be analyzed.
An example of the prior art technology for detecting fine defects on the surface of the inspection object such as the semiconductor substrate is described in U.S. Pat. No. 5,798,829. This technology fixedly irradiates a condensed laser luminous flux to the surface of the semiconductor substrate to form an illumination spot having a predetermined size, detects scattered light from a foreign matter occurring when the foreign matter passes through the illumination spot when it adheres to the surface of the semiconductor substrate, and inspects the foreign matters and the defects on the entire surface of the semiconductor substrate.
In this case, scattered light (hereinafter called “background scattered light”) always occurs in the illumination spot owing to fine surface roughness (micro-roughness) on the semiconductor wafer even when the foreign matters ad the defects do not pass. It is known that shot noise originating from this background scattered light is generally predominant in a noise component of detection signals when fine foreign matters are detected. Since the shot noise is proportional to the square root of light intensity as the base of the noise, the noise level becomes greater substantially in proportion to the square root of the intensity of the background scattered light when the fine foreign matters are detected.
It is well known that when a semiconductor wafer surface is illuminated by P polarization from a low angle of elevation such as a Brewster's angle for a Si crystal in the case of fine foreign matters following the rule of elastic scattering (Rayleigh scattering), on the other hand, scattered light from the foreign matter does not have a strong directivity in an azimuth direction but is scattered in all azimuth directions at a substantially equal intensity.
Background scattered light originating from surface roughness of a semiconductor wafer to which finish polishing is sufficiently made does not exhibit an extremely strong directivity in the azimuth direction, either. In such a case, therefore, it is preferred to uniformly condense and detect the scattered light dispersing in all the azimuth directions in order to secure a high S/N ratio of detection signals. An optical detector of the prior art technology described above collectively receives all of scattered light in the azimuth directions and a preferred S/N ratio can be acquired in this case.
However, the background scattered light originating from the surface roughness (micro roughness) of the semiconductor wafer has in some cases strong directivity. For example, it is known that the background scattered light originating from the surface roughness exhibits strong directivity owing to the correlation between the crystal direction and the illumination direction in epitaxial wafers.
In this case, the output signal of the optical detector that detects the background scattered light originating from the surface roughness in a strong azimuth direction contains greater noise components. Obviously, therefore, it is not advantageous to equally handle the scattered light signal detected at a strong azimuth angle of the background scattered light originating from the surface roughness with the scattered light signal detected at a weak azimuth angle of the background scattered light originating from the surface roughness.
On the other hand, U.S. Pat. No. 7,002,677 describes a technology for partially cutting off scattered light traveling in a specific direction and states that an S/N ratio of a scattered light signal from a foreign matter/defect of an inspection object can be improved by conducting control in such a fashion that an azimuth angle direction in which the background scattered light is partially cut off and an optical detector can receive the scattered light in only an azimuth angle direction in which the background scattered light is weak.